Re: Parsing Ratio with zero denominator

On Sun, 31 May 2020, Dannyu NDos wrote:

I mean, shouldn't there be a guard?

instance (Integral a, Read a) => ReadPrec (Ratio a) where readPrec = parens . prec 7 $ do n <- step readPrec lift (expect (Symbol "%")) d <- step readPrec guard (0 /= d) return (n % d)

I think you are right. A readPrec parser should only generate parser errors and should be total. I do not know if anyone relies on the current behavior. I suspect that most users are not aware of the problem and their programs will be aborted in case of zero denominator where they shouldn't.